1. Field of the Invention
This invention relates to an opto-magnetic recording medium capable of recording or reproducing information by a light beam being applied thereto.
2. Description of the Prior Art
As opto-magnetic recording mediums, there are known mediums having as a recording layer a polycrystal thin film such as MnBi or MnCuBi, an amorphous thin film such as GdCo, GdFe, TbFe, DyFe, GdTbFe or TbDyFe, or a single crystal thin film such as TbFeO.sub.3.
Of these thin films the amorphous thin film has recently been considered to be excellent as the recording layer of the opto-magnetic recording medium when the film forming property with which a thin film of a great area is made at a temperature in the vicinity of room temperature, the writing efficiency for writing signals with small opto-thermal energy and the reading-out efficiency for reading out the written signals at a good S/N ratio are taken into account. Particularly, the amorphous thin film GdTbFe has a great Kerr rotation angle and a curie point of about 150.degree. C. and is therefore suitable as the recording layer. Further, as a result of the study continued in order to improve the Kerr rotation angle, it has been found that the amorphous thin film GdTbFeCo has a sufficiently great Kerr rotation angle and permits reading-out of a good N/S ratio.
Generally, however, magnetic materials used for the magnetic recording layer, including the amorphous magnetic alloys such as GdTbFe, have a disadvantage that they are poor in corrosion resistance. That is, when they are brought into contact with the atmosphere or steam, their magnetic characteristic is reduced and finally, they are comptetely oxidized and become transparent.
To eliminate such a disadvantage, there has heretofore been proposed a disc-like recording medium in which a protective layer formed of a dielectric material such as SiO.sub.2, SiO or Si.sub.3 N.sub.4 transmitting therethrough the recording light and the reproducing light is provided on each side of the magnetic recording layer and further, the magnetic recording layer is enveloped by inert gas.
As an example of such conventional opto-magnetic recording medium, there is known a medium in which, as shown, for example, in FIG. 3 of the accompanying drawings, a protective layer 2 formed of the dielectric material as previously mentioned is formed on a writing side substrate 1a and a magnetic recording layer 3, a spacer layer 4 and a reflection layer 5 are successively provided on the protective layer 2 and further the reflection layer 5 is cemented to a protective substrate 1b with an adhesive layer 6 interposed therebetween. However, where the magnetic recording layer 3 is as thin as several hundred .ANG., the magnetic characteristic thereof has sometimes been deteriorated in spite of the presence of the protective layer 2 and the spacer layer 4 if the medium is placed in a high humidity state for a long time. This is because, when the recording sensitivity and the reproducing efficiency are taken into account, the protective layer 2 and the spacer layer 4 cannot be formed to a sufficient thickness to completely prevent transmission of moisture or oxygen therethrough. The adherence property of the protective layer 2 such as Si.sub.3 N.sub.4 with respect to the substrate 1a cannot be said to be satisfactory particularly where the substrate 1a is formed of a plastic material, and peel-off or crack has thus occurred and this has led to a problem in respect to durability.